// A fast random []byte generator. The default Source is used, which
// is safe for concurrent use by multiple goroutines. The default Source
// is seeded at package init.
package rndstr

import (
	"math/rand"
	"time"
)

func init() {
	rand.Seed(time.Now().UnixNano())
}

const letterBytes = "1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_"
const (
	letterIdxBits = 6                    // 6 bits to represent a letter index
	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
)

// Fills the []byte with random characters
func PutBytes(b []byte) {
	n := len(b)
	for i, cache, remain := n-1, rand.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = rand.Int63(), letterIdxMax
		}
		idx := int(cache & letterIdxMask)
		b[i] = letterBytes[idx]
		i--
		cache >>= letterIdxBits
		remain--
	}
}

// Returns []byte of 'n' length filled with random characters
func GetBytes(n int) []byte {
	b := make([]byte, n)
	PutBytes(b)
	return b
}